The instant invention pertains to apparatus for fluid catalytic cracking of hydrocarbons and more particularly to such apparatus in which a catalyst reactor vessel is positioned directly over a catalyst regenerator vessel.
In fluid catalytic cracking units, it is known that atomization of an oil feedstock stream as it is injected into the riser reactor for mixing with catalyst therein enhances cracking and thereby increases conversions. This is so because increased atomization increases the surface area of the oil and thereby increases heat transfer to the oil. Such heat transfer first vaporizes and thereafter cracks the oil. Atomization is desirable when converting low boiling point oils since more efficient vaporization and cracking would permit a shorter riser reactor and/or better conversions and/or a reduction in the severity of operating conditions of the fluid catalytic cracking unit.
Atomization of the oil as it enters the riser reactor in a fluid catalytic cracking unit is particularly important when converting heavy oils which have high boiling temperatures, generally in the range of 600.degree. F. to 1200.degree. F. Heavy oils are especially difficult to crack to valuable products because their high boiling point and viscosity make satisfactory vaporization very difficult.
Heavy oils can be successfully cracked into desirable products where they have been vaporized prior to contact with the catalyst. With conventional feeds, vaporization is achieved by radiant energy transfer from the hot cracking catalyst to the feed droplets. This type of vaporization mechanism is satisfactory for oils boiling below thermal cracking temperatures which commence at about 850.degree. F. For heavy oils however vaporization of large droplets by heat transfer is not completed prior to the onset of thermal cracking and coke formation. Coke laydown is worsened where liquid oil strikes the hot catalyst particles.
There is a prior art catalytic cracking unit, which includes an apparatus for atomizing oil as it is injected into the riser reactor of a fluid catalytic cracking unit having a catalyst regenerator vessel located adjacent a catalyst reactor vessel. It would be desirable to provide for atomization of oil entering a riser reactor in a fluid catalytic cracking unit having stacked vessels, i.e., a catalyst reactor vessel positioned directly over a catalyst regenerator vessel, such as the type manufactured by the M. W. Kellog Co. and sold under the Orthoflow name.
The prior art cracking unit atomizer comprises a lift pot having a nozzle cartridge assembly positioned therein located at the lower end of the riser reactor. A pipe provides fluid communication between the lift pot and the catalyst regenerator vessel. Oil feedstock is provided to the cartridge assembly which atomizes the same and mixes the catalyst therewith at the lower end of the riser reactor. It would be desirable to provide an additional conduit between the catalyst regenerator vessel and the lift pot to increase control of the catalyst flow thereto.
In one aspect, the invention comprises a fluid catalytic cracking unit having a catalyst reactor vessel positioned directly over a catalyst regenerator vessel. Spent catalyst is communicated from the reactor vessel to the regenerator vessel through a standpipe which connects the two. A mixing chamber for mixing catalyst and oil feedstock is positioned beneath the lowermost level of the catalyst regenerator vessel. The mixing chamber and the lower portion of the regenerator vessel are in fluid communication via a conduit which connect the two. Means are provided for injecting an oil feedstock into the mixing chamber which has one end of a riser reactor in fluid communication therewith. The other end of the riser reactor communicates with the reactor vessel. The invention may be embodied in fluid catalytic cracking units having both external and internal riser reactors.
In another aspect of the invention, a second conduit is provided between the mixing chamber and the lower portion of the regenerator vessel and means are provided for independently regulating the flow of catalyst in each of the conduits which connect the regenerator vessel with the mixing chamber.